1. Field of the Invention
This invention pertains to processing of nuclear reactor fuel elements, and more particularly to methods and apparatus for removing nuclear fuel from clad fuel elements.
2. Description of the Prior Art
In today's commercial nuclear reactors, a fissile material, for example enriched uranium, is employed as a nuclear fuel. Fissioning of this material in the reactor produces heat from which electricity is ultimately generated. A typical large nuclear reactor contains approximately 193 fuel assemblies, each fuel assembly containing 208 individual fuel elements. A fuel element, as referred to herein, means a metallic tube, called "cladding", of zircoloy, stainless steel or other material which contains nuclear fuel pellets. Within each of these fuel elements, in the example of the reactor described, there are as many as 240 fuel pellets which comprise the fissile material. Hence, a nuclear reactor contains approximately 9.6 million fuel pellets, at least a portion of which are replaced on a yearly basis approximately.
It is therefore apparent, that the nuclear fuel industry processes a large number of new fuel pellets and a correspondingly large number of new fuel elements to replace those consumed during reactor operation. Moreover, spent fuel, which is the name commonly given to used fuel removed from a reactor, is still valuable in that it may be reprocessed for further use in another reactor at a later date.
In order to reprocess spent fuel, it is first necessary to remove the fuel pellets from the metallic sheath cladding within which they are contained. This is neither a simple nor a fast operation as the spent fuel is highly radioactive and must be handled remotely. Also, the diametral clearance between the pellet outer diameter and the fuel tube or cladding inner diameter is relatively small, nominally of the order of 0.008 inches at assembly. Therefore, there is a high probability that the pellets will bind within the cladding thereby eliminating an obvious removal technique such as pushing the pellets out of the cladding. This binding problem is further aggravated as a result of reactor operation which causes local deformation and shrinkage of the cladding as well as the possibility of diametral growth of the fuel pellets. Together, these effects of reactor operation reduce the already small clearance between the pellets and the cladding and make removal of the fuel more difficult.
Although normal processing of new nuclear fuel elements involves the insertion of fuel pellets into the cladding, there are circumstances where it is necessary to remove fuel pellets from the cladding of new fuel elements. One example is when the fuel element is found during manufacture to be defective in that it doesn't meet the high quality standards required of fuel elements for nuclear reactors. Another circumstance might be where fuel element design has been deemed obsolete prior to reactor use and it is desirous to replace these fuel elements by those of a new design. While removal of fuel pellets from new fuel elements is not as difficult or time consuming as removal of fuel pellets from spent fuel elements, the removal process is still somewhat difficult because of the close clearance involved. Binding of the pellets within the cladding is still a problem; again therefore, the pellets cannot be simply pushed out of the cladding.
In the prior art, the fuel pellets were removed from fuel elements by a method which involved cutting a fuel element crosswise into a number of short sections. The cut sections of fuel element are then placed in a solution which dissolves the metal cladding but does not affect the fuel pellets. Although effective, there are a number of disadvantages to this prior art method. One disadvantage is that the fuel pellets are severely damaged by this removal process and thus, require additional reprocessing prior to reuse. Another disadvantage is that relatively large amounts of fuel dust are generated and this must be specially handled for recycling.
In more recent prior art, the above disadvantages were overcome by a method utilizing mechanical slitting of the cladding into two halves, then separating the two halves and causing the fuel pellets to drop into a receptacle. Unfortunately, a number of new disadvantages arose. It has been shown that the splitting rates are not high and the process involves extensive labor in setting up the equipment, operating it and maintaining the cutting tools.
In the past, therefore, at least two different methods were utilized for removing the fuel pellets from clad fuel elements. However, each of these methods has certain disadvantages which render these methods somewhat unsatisfactory.